121 MOEPHOLOGY AND EVOLUTION
anterior to the posterior clinoid processes. The sella turcica is shallow or absent, but the surface for the pituitary body appears like a platform raised above the deep lateral parts of the middle fossa.
The tentorial ridge extends back to the internal occipital protuberance; it does not stop at the back of the petrosals as in the Orang. The posterior cranial fossa is deepest laterally, where its floor is grooved for the lateral sinuses; and either sinus may be the larger. The fossa is bounded behind by the occipital plane of the skull, and as the foramen magnum is slightly extended on to this plane the posterior part of the fossa is small and flat. This feature, which is common to all the Apes, distinguishes them from Man.
A line drawn back along the cribriform plate, if raised up to the level of the mouth of the pit between the orbits, will be continuous with one drawn along the basi-sphenoid and basi-occipital.
Keifeh (100) points out that the average cranial capacity of ten females is 364 c.c., and of sixteen males 405 c.c. But the cranial capacity does not help us to distinguish between the different varieties of Chimpanzee.
Mandible.—The ramus is broad, but not so large as in Siniia. On its upper border a shallow sigmoid notch separates the sharp, recurved coronoid process from the condyle. The inclination of the ramus and body vary, being more vertical in the young animal. The outline of the lower border of the body varies considerably with age and sex. The symphysis slopes backwards; towards the lower part of its posterior surface there is a genial fossa in which the genio-hyoid and genio-glossus